Support for zip files using backslashes as the path separator.

This adds support for zip files that use backslashes as the path separator instead of following the spec as they should...

I am including an example zip file that has backslashes (copied from here: https://github.com/pathikrit/better-files/issues/262).

Extracting all or just selected files, adding/removing to/from the zip and viewing the file/directory tree all seem to be working correctly.